Analog floating-gate atmometer

ABSTRACT

An atmometer system based on an analog floating-gate structure and circuit. The floating-gate circuit includes a floating-gate electrode that serves as a gate electrode for a transistor and a plate of a storage capacitor. A conductor element exposed at the surface of the integrated circuit is electrically connected to the floating-gate electrode; reference conductor elements biased to ground are also at the surface of the integrated circuit. In operation, the transistor is biased and moisture is dispensed at the surface. The drain current of the transistor changes as the floating-gate electrode discharges via the surface conductors and a conduction path presented by the moisture. The elapsed time until the drain current stabilizes indicates the evaporation rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority to U.S. patentapplication Ser. No. 14/749,875, filed Jun. 25, 2015, which claimspriority, under 35 U.S.C. § 119(e), of U.S. Provisional Application No.62/018,248, filed Jun. 27, 2014, incorporated herein by this reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

This invention is in the field of electronic sensors. Embodimentsdisclosed in this specification include electronic sensors for sensingevaporation rate and relative humidity.

Atmometers are instruments for measuring the rate at which waterevaporates from a wet surface into the atmosphere. Evaporation rate andrelated parameters are of particular importance in agriculture, amongother industries. For example, knowledge of the rate at which plantstranspire can assist the scheduling of irrigation activities, both intiming and in the amount of water applied to the crops. Efficient use ofavailable water is, of course, especially important in arid regions, orthose regions experiencing drought, where ground and surface water is ata premium.

Conventional atmometers operate by measuring the rate at which water isdrawn from a reservoir to a surface exposed to the atmosphere. In onetype of conventional atmometer, the exposed surface is a porous ceramicplate that is connected to the water reservoir by a tube. In theagricultural context, a canvas cover is typically provided over theceramic plate to mimic the canopy of the crop of interest. As waterevaporates from the ceramic plate, additional water is drawn through thetube from the reservoir to the plate. Measurement of the water level inthe reservoir over time thus provides a measurement of the rate at whichwater is evaporating at the ceramic plate, from which the transpirationrate of the crop plant of interest can be inferred. The frequency atwhich measurements can be obtained from those conventional atmometers isnecessarily limited, and as such these measurements are each essentiallyaveraged over relatively long time periods (e.g., at least a few hours).In addition, because the evaporation rate measurement is typicallyobtained by visual inspection of the reservoir level, these conventionalatmometers are not conducive to automation.

A recent trend, however, is the increasing deployment of networkedcommunications among computer systems and other electronic devicesthemselves, absent human initiation or control of the communications.These machine-to-machine (“M2M”) communications are now being carriedout over a wide-area network, such a network now often referred to asthe “Internet of Things” (“IoT”). In this context, the nature of thecommunications can differ significantly from conventional human-orientedInternet communications. The amount of data transmitted from one“machine” to another in a given transmission is often quite small (e.g.,streaming video is not often involved), and is often not particularlytime-sensitive. As such, the communications requirements for IoT can besomewhat relaxed. On the other hand, the number of M2M network nodes inthe future is contemplated to be substantially larger than the number ofnodes in the human-oriented Internet.

By way of further background, humidity is an important parameter in manyindustries, such as semiconductor processing, pharmaceutical and otherchemical processing, petroleum refining, paper and textile production,agriculture, medicine, and food processing, to name a few. As such,conventional humidity sensors are used in equipment for these and otherindustries, examples of such equipment including respiratory equipment,sterilizers, incubators, ovens, dryers and dessicators, condensationprevention equipment, and monitoring equipment such as soil moisturemonitors and building environmental control.

By way of further background, Gu et al., “Kinetics of Evaporation andGel Formation in Thin Films of Ceramic Precursors”, Langmuir, Vol. 30,No. 48 (American Chemical Society, 2014), pp. 14638-47 (see “SupportingInformation for Kinetics of evaporation and gel formation in thin filmsof ceramic precursors”, available athttp://www.clemson.edu/ces/kornevlab/article/43si.pdf), describes theevaporation mechanism as a diffusion mechanism that depends on the watervapor concentration gradient between the surface of the evaporatingwater droplet (i.e., at 100% relative humidity) and the ambientatmosphere (i.e., at the ambient relative humidity). This mechanism isexpressible as a temperature-dependent diffusion equation, from whichthe ambient relative humidity can be determined from measurements of theevaporation rate and the temperature.

BRIEF SUMMARY

Disclosed embodiments provide a device, system, and method for rapidlyand frequently measuring evaporation rate.

Disclosed embodiments provide such a device, system, and method that canbe implemented as an automated sensor and deployed in amachine-to-machine (M2M) networked system.

Disclosed embodiments provide such a device, system, and method suitablefor rapidly and frequently measuring relative humidity.

Disclosed embodiments provide such a device that can be fabricated usingconventional integrated circuit manufacturing technology, and thus atlow cost.

Disclosed embodiments provide such a device, system, and method that canbe calibrated and thus provide repeatable and reliable measurements.

Other objects and advantages of the disclosed embodiments will beapparent to those of ordinary skill in the art having reference to thefollowing specification together with its drawings.

According to certain embodiments, an integrated circuit including ananalog floating-gate structure is arranged in a system for sensingevaporation rate. A floating-gate electrode serves as a gate of ametal-oxide-semiconductor (MOS) transistor and is connected to aconductor element, for example in the form of a metal pad, that isdisposed at the surface of the integrated circuit. One or more referenceconductor elements are also disposed at the surface, separated from theconductor element coupled to the floating-gate electrode, and biased toa reference voltage such as ground. With the drain and gate of thetransistor biased so that drain current is conducted, moisture isdispensed at the surface of the integrated circuit. The drain current ismonitored over time, as charge on the floating-gate electrode isdischarged to the reference conductor electrodes via the dispensedmoisture. The evaporation rate of the moisture can be inferred from thetime elapsed from dispensing of the moisture until the drain currentreaches a steady-state equilibrium.

According to an embodiment, the analog floating-gate structure isimplemented into a system that also includes control logic forcontrolling the bias and operation of the analog floating-gate structureto obtain the evaporation rate measurement, along with a mechanism fordispensing moisture at the integrated circuit surface.

According to an embodiment, the system also includes processor circuitrythat calculates relative humidity of the ambient atmosphere by combiningthe measured evaporation rate and a temperature measurement.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1a is an electrical diagram, in block form, of a sensor andcontroller node as may be deployed in a distributed network system,according to some embodiments.

FIG. 1b is an electrical diagram, in block form, of an atmometer sensorin the node of FIG. 1a according to embodiments.

FIGS. 2a and 2c are plan views, and FIGS. 2b and 2d are cross-sectionalviews, of an analog floating-gate integrated circuit implemented in theatmometer sensor of FIG. 1b according to embodiments.

FIG. 3a is an electrical diagram, in schematic form, of an equivalentcircuit for the analog floating-gate structure of FIGS. 2a through 2daccording to embodiments.

FIG. 3b is a timing diagram illustrating the operation of the analogfloating-gate equivalent circuit of FIG. 3 a.

FIG. 4 is a flow diagram illustrating the operation of the atmometersensor node of FIGS. 1a and 1b in measuring evaporation rate and,optionally, relative humidity according to embodiments.

DETAILED DESCRIPTION

The one or more embodiments described in this specification areimplemented into an atmometer system or device implemented in anetworked arrangement, such as according to the “Internet of Things”(IoT), as it is contemplated that such implementation is particularlyadvantageous in that context. However, it is also contemplated thatconcepts of this invention may be beneficially applied to in otherapplications, such as stand-alone devices or as integrated intomanufacturing, environmental, or other equipment for which measurementof evaporation rate is useful. Accordingly, it is to be understood thatthe following description is provided by way of example only, and is notintended to limit the true scope of this invention as claimed.

As mentioned above in the Background of the Invention, it iscontemplated that distributed networked systems consisting of a numberof sensors and controllers that each contain significant computationalcapacity and are capable of M2M communication with one another will bewidely deployed over the coming years. In these networks, the number ofnodes (i.e., the sensors, controllers, or both) in such a network canrange from several nodes to thousands of nodes, depending on theparticular application. These networks have become attractive in thecontexts of facilities management (e.g., for environmental control andsecurity management) and industrial control (e.g., control of motors andvalves). In particular, it is contemplated that the embodimentsdescribed in this specification may be particularly useful inagricultural, environmental, and manufacturing contexts.

FIG. 1a illustrates, by way of example, the high-level architecture of asensor node N into which embodiments of this invention may beimplemented, by way of example. Node N is intended for deployment into adistributed networked system, along with nodes in that systemconstructed by similar or alternative architectures, perhaps dependingon the function. It is contemplated that those skilled in the art havingreference to this specification will be readily able to implement theappropriate hardware and software for realizing embodiments of thisinvention as suitable for particular applications, without undueexperimentation.

Node N in this embodiment of the invention corresponds to a programmablesubsystem including embedded microcontroller unit (MCU) 2 in combinationwith various peripheral functions. For example, node N may be physicallyrealized by way of a single circuit board on which MCU 2 will bemounted, along with other integrated circuits and discrete components asdesired, housed in an appropriate housing or enclosure suitable for itsenvironment. Alternatively, node N may be realized by way of multiplecircuit boards, as a single integrated circuit, or as a part of a largerelectronic system, depending on its functionality. In the architectureof node N of FIG. 1a , communication with other nodes and perhaps with ahost computer system are carried out by way of wireless transceiver 4,constructed and operating in the appropriate manner for the particularcommunications facility being used. If communication is to be carriedout wirelessly, any one of a number of conventional protocols andphysical layer standards, including IEEE 802.11a/b/g/n etc., Bluetooth,and Bluetooth 4.0 (i.e., Bluetooth Low Energy, or “BLE”), may serve asthat communications facility; alternatively, transceiver 4 may beconfigured for communication over Ethernet or another type of wirednetwork.

According to this embodiment, node N also includes one or moreinput/output functions for interacting with the physical environmentexternal to that node. One such function is atmometer sensor 5, which iscoupled to and controlled by MCU 2. Optionally, control output circuit 7may also be provided in node N, coupled to and controlled by MCU 2 torealize a controller function. Examples of control output circuit 7include analog output driver circuitry, serial and parallel digitaloutputs, pulse-width-modulated (PWM) output driver circuitry, drivercircuitry for an alarm or an annunciator, and LED drivers, to name afew. The particular numbers and functions of input/output functions(e.g., atmometer sensor function 5 and control output circuit 7) dependon the conditions and operations that node N is to carry out in thenetworked system. Such additional sensor and controller functions may beadditional instances of the same function, or may be configured as otherfunctions. Other sensor functions that may additionally be realized atnode N include temperature sensors, motion sensors, humidity sensors,transducers of various types as suitable in industrial instrumentation,cameras, thermal imaging sensors, photosensors, and the like.

In the example of FIG. 1a , node N also includes power manager function8, which controls the powering of the various functions within the node.For example, node N may be powered by any one or more sources includingwired power (e.g., power over USB, DC output from a rectifier ormicro-grid), battery power, solar power, wireless power transfer (e.g.,over the wireless communications facility or separately), and the like.

In this embodiment, MCU 2 in node N is configured to include certainfunctions particular to the construction and operation of thisembodiment of the invention, for example by way of logic circuitryprogrammed to execute program instructions stored in memory resource 12or received over the communications facility via wireless transceiver 4.For example, at least a portion of this programmable logic isrepresented by ALU 10, which operates in combination with memoryresource 12 that is also implemented within MCU 2 in this example. Insome embodiments, MCU 2 is realized by any one of a number ofmicrocontroller or microprocessor devices available in the industry,examples of which include those of the C2xxxx and CORTEX microcontrollerfamilies available from Texas Instruments Incorporated. Othermicrocontrollers and microprocessors of similar computational capacity,or custom logic circuitry, may alternatively be used for MCU 2, so longas adequate computational capacity is provided. It is contemplated thatthose skilled in the art having reference to this specification will bereadily able to select and implement the appropriate device or circuitryfor use as MCU 2 for the particular application.

In this architecture, memory resource 12 stores both programinstructions executable by ALU 10, and also data upon which ALU 10carries out those program instructions. Memory resource 12 can berealized by one or more memories within MCU 2 or external to MCU 2, andby a variety of memory technologies, including either or both ofvolatile memory (e.g., static random-access memory) and non-volatilememory (e.g., flash memory). Program and data memory may occupy separatememory address spaces, or may be contained within a single memory space.For the example of MCU 2 implemented as a C2xxx microcontroller, amodified Harvard architecture is employed by way of which program anddata occupy separated regions of a global memory address space, but canbe accessed by way of separate hardware pathways.

Node N and MCU 2 are also contemplated to include other circuitry andfunctions beyond those shown in FIG. 1a , such other circuitry andfunctions suitable to its functionality as a stand-alonemicrocontroller. Examples of such other circuitry and functions inputand output drivers, analog-to-digital converters, digital-to-analogconverters, clock circuits, voltage regulators, among others. Thesecircuits may be also be involved in the operation and execution ofprogram instructions by MCU 2 and the other functions of node N. It iscontemplated that those skilled in the art having reference to thisspecification will readily comprehend other necessary support circuitryincluded within MCU 2.

Referring now to FIG. 1b , the architecture of atmometer sensor 5according to an embodiment will now be described. In this example,atmometer sensor 5 is arranged in the form of a subsystem, includingintegrated circuit 20 in which a floating-gate circuit is implemented,in combination with other functions such as moisture dispenser 30,optional heater 31, and control circuitry 35. As evident from FIG. 1b ,control circuitry 35 is coupled to various nodes of integrated circuit20, to moisture dispenser 30, and to heater 31, and will apply controlsignals and monitor electrical parameters to carry out the functionsdescribed in further detail below, those functions including thedispensing of moisture onto the surface of integrated circuit 20 bymoisture dispenser 30, the bias and monitoring of components ofintegrated circuit 20, and the optional heating of the surface ofintegrated circuit 20 by heater 31. Control circuitry 35 may be realizedin whole or in part as separate circuitry deployed within the subsystemof atmometer sensor 5 and in communication with MCU 2, or within MCU 2itself. In some realizations, all or part of control circuitry 35 may beimplemented into integrated circuit 20 itself, along with the analogfloating-gate device. In any case, it is contemplated that those skilledin the art having reference to this specification will be readily ableto implement control circuitry 35 in the manner appropriate for carryingout the functions described herein, using the available and appropriatetechnology for the particular architecture to be used.

Integrated circuit 20 includes an analog floating-gate circuit andstructure in connection with which embodiments of this invention may beused. This analog floating-gate circuit includes an electricallyfloating electrode, namely floating-gate electrode 22 in the arrangementof FIG. 1b , that serves multiple functions. One function of analogfloating-gate electrode 22 in this circuit of FIG. 1b is as a plate ofstorage capacitor 26. According to this embodiment, another plate ofstorage capacitor 26 receives a gate voltage VG from control circuitry35 at terminal G, such that charge can be externally applied to andstored by storage capacitor 26. Another function of floating-gateelectrode 22 is as the gate of metal-oxide-semiconductor (MOS)transistor 24. In the example of FIG. 1b , the drain of transistor 24 atdrain terminal D receives drain voltage VD from control circuitry 35,and the source of transistor 24 at source terminal S is biased to areference voltage, namely ground in this example. In this arrangement,gate voltage VG applied at terminal G by control circuitry 35 willcapacitively couple to floating-gate electrode 22 via storage capacitor26, with that voltage establishing the voltage at the gate of MOStransistor 24, and thus the extent to which transistor 24 conductsbetween its drain D and its source S for a given drain-to-source voltageVD.

In this arrangement, floating-gate electrode 22 also serves as a plateof each of tunnel capacitors 28 p, 28 n. Tunnel capacitors can applycharge to or remove charge from floating-gate electrode 22,“programming” it to a particular analog state. In the example ofintegrated circuit 20, the plate of tunnel capacitor 28 p opposite thatof electrode 22 is connected to a terminal TP, while an opposing plateof tunnel capacitor 28 n is connected to a terminal TN. The capacitordielectric for tunnel capacitors 28 p, 28 n is contemplated to berelatively thin, to allow mechanisms such as Fowler-Nordheim tunnelingto transfer charge between terminals TP, TN and floating-gate electrode22, depending on the bias. While, as noted above, tunnel capacitors 28p, 28 n permit both the programming of stored charge onto floating-gateelectrode 22 and the removal of that charge (“erase”), it iscontemplated that only one of these tunnel capacitors 28 p, 28 n may beimplemented in some implementations.

In its general operation as an analog floating-gate device, the“programming” of floating-gate electrode 22 is carried out byapplication of a pulse of an appropriate negative voltage to terminal TNrelative to the voltage at terminal TP and to the ground referencevoltage at the opposite plate of storage capacitor 26, to causeelectrons to tunnel through tunnel capacitor 28 n. Because of thevoltage divider formed by capacitors 28 n, 28 p, 26, most of thatprogramming voltage will appear across tunnel capacitor 28 n, enablingelectrons to tunnel through its capacitor dielectric to analogfloating-gate electrode 22, and become trapped at floating-gateelectrode 22. Conversely, electrons can be removed (“erased”) fromfloating-gate electrode 22 by applying an appropriate positive voltageat terminal TP relative to terminal TN and to the ground referencevoltage at the opposite plate of storage capacitor 26. Again, thevoltage divider of capacitors 28 n, 28 p, 26 will result in most of thatvoltage appearing across tunnel capacitor 28 p, causing electrons thatare trapped on floating-gate electrode 22 to tunnel through itscapacitor dielectric to terminal TP. In the analog sense, the durationof the program and erase pulses can be adjusted to precisely set thecharge state at floating-gate electrode 22. Following programming anderasure, as the case may be, the extent to which charge is trapped onfloating-gate electrode 22 will establish a voltage across storagecapacitor 26, and thus a gate voltage for MOS transistor 24 thatcontrols its conduction.

As evident from the above description and from FIG. 1b , controlcircuitry 35 is coupled to the various terminals of the floating-gatedevice of integrated circuit 20, including to terminals G, D, TP, TN soas to apply corresponding voltages VG, VD, VTP, VTN, respectively. Inthis example, control circuitry 35 monitors the drain current IDconducted by transistor 24, for example by detecting the current drawnby its bias of drain voltage VD to directly measure drain current ID.Alternatively, transistor 24 may drive an analog circuit or otherfunction, such as an amplifier, from which control circuitry 35 obtainsa measurement of drain current ID of transistor 24. Also according tothis embodiment, control circuitry 35 also includes a timer function,such as a clocked counter or other conventional timer, to provide a timebase for the monitoring of drain current ID over time, as describedbelow.

In this embodiment as shown in FIG. 1b , integrated circuit 20 alsoincludes conductor element 21 that is electrically connected tofloating-gate electrode 22, and reference conductor element 25 that iscoupled to a reference voltage, such as ground, but is otherwiseelectrically isolated from conductor element 21 and the other nodes ofthe analog floating-gate device in integrated circuit 20. As will beevident from the following description, these conductor elements 21, 25are constituted by metal pads at the surface of integrated circuit 20that are exposed to the ambient environment of the device.

FIG. 2a illustrates, in plan view, an example of the construction of ananalog floating-gate structure such as may be used to implement thecircuit of FIG. 1b , and in connection with which embodiments may beused. FIGS. 2b and 2d illustrate certain elements in that structure incross-section. It is contemplated that this floating-gate structure maybe fabricated by way of conventional manufacturing technology, includingat those process nodes extending into the sub-micron regime. It istherefore contemplated that those skilled in the art having reference tothis specification will be readily able to adapt the structure of FIGS.2a through 2d in the desired manufacturing technology, without undueexperimentation.

As shown in FIG. 2a , floating-gate electrode 22 is constructed ofpolycrystalline silicon (polysilicon) element 36, which extends over thesurface of a semiconductor wafer (or over a semiconductor surface layer,in the silicon-on-insulator context) in forming multiple devices orcomponents. Polysilicon element 36 is typically doped to a desiredconductivity type and concentration, to be conductive to the desiredextent; for example by way of n-type doping for this example in whichMOS transistor 24 is n-channel. Polysilicon element 36 has a widenedportion at its end serving as a lower plate of storage capacitor 26, andis otherwise narrower, for example at a minimum feature size for themanufacturing technology. The cross-section of storage capacitor 26shown in FIG. 2b shows the lower plate portion of polysilicon element 36as overlying trench isolation dielectric structure 33. Gate dielectric37, for example formed of deposited or thermal silicon dioxide, isdisposed between the surface of isolation dielectric structure 33 andpolysilicon element 36, and will also underlie polysilicon element 36 atthose locations at which it overlies active regions (i.e., at transistor24 and tunnel capacitors 28 p, 28 n). In this example, the surface intowhich isolation dielectric structure 33 is formed is the top surface ofp-type silicon substrate 29. Upper plate 32 of storage capacitor 26 isformed of a metal such as tantalum nitride, and overlies the widenedportion of polysilicon element 36 at this location. In this embodimentof the invention, capacitor dielectric 38 is formed of one or moredielectric layers, for example silicon nitride, silicon dioxide, or acombination of these or other dielectric materials.

As shown in FIG. 2a , transistor 24 and tunnel capacitors 28 p, 28 n areconstructed along those portions of polysilicon element 36 that overlieactive regions (i.e., that do not overlie isolation dielectricstructures 33). Specifically, MOS transistor 24 is defined at thatportion of polysilicon element 36 overlying an active region of p-typesubstrate 29, with gate dielectric 37 disposed between polysiliconelement 36 and that active region. The source and drain of transistor 24are formed by heavily-doped n-type source/drain regions 35 _(n)implanted and diffused into the p-type active region on opposite sidesof polysilicon element 36 in the conventional self-aligned fashion. Topside contacts from an overlying metal conductor, and corresponding toterminals D, S as in the circuit of FIG. 1b , are made through aninterlevel dielectric layer to source/drain regions 35 _(n).

Tunnel capacitors 28 n, 28 p are constructed in the conventional mannerfor floating-gate devices. In this embodiment, tunnel capacitor 28 n isconstructed essentially similarly as n-channel MOS transistor 24, butwhere polysilicon element 36 overlies an instance of an isolated p-typewell, for example a p-well isolated from the underlying substrate by aburied n-type layer and an n-well ring. Gate dielectric 37 is formedbetween polysilicon element 36 and the surface of the p-well to serve asthe capacitor dielectric, and heavily-doped n-type source/drain regions35 _(n) are formed into the isolated p-well in a self-aligned manner.Terminal TN is connected via a top-side contact to the isolated p-wellin which these source/drain regions 35 _(n) are formed, so tunnelcapacitor 28 n operates as a capacitor rather than a transistor, butwith source/drain regions 35 _(n) serving as sources of electrons when anegative bias is applied to terminal TN. Tunnel capacitor 28 p isconstructed essentially similarly as tunnel capacitor 28 n, but at alocation at which polysilicon element 36 overlies gate dielectric 37 atthe surface of an n-well formed into substrate 29. Terminal TP isconnected to this n-well by a top-side contact, and p-type source/drainregions 35 _(p) that are formed on either side of polysilicon element 36act as a sources of holes when a positive bias is applied to terminalTP. Tunnel capacitors 28 p, 28 n may of course be constructed accordingto other arrangements as suitable for particular implementations andmanufacturing technologies.

In the example shown in FIG. 2a , the difference in relative areabetween tunneling capacitors 28 p, 28 n, on one hand, and storagecapacitor 26, on the other hand, along with any differences in thecapacitor dielectric materials and thicknesses, will be reflected in therelative capacitances between these elements. Because the capacitance ofstorage capacitor 26 is substantially larger than the capacitances oftunnel capacitors 28 n, 28 p (and also the parasitic gate-to-activecapacitance of transistor 24), tunneling of electrons can be achieved atreasonable bias voltages to avoid damage or breakdown. This disparity incapacitive coupling is contemplated to provide excellent programming anderase performance.

Many variations in the electrical and physical construction of an analogfloating-gate circuit in an integrated circuit, relative to thatdescribed above, are contemplated. From an electrical standpoint, suchvariations include circuits such as a reference circuit arranged as adual floating-gate differential amplifier circuit, as known in the art.As mentioned above, examples of other analog floating-gate circuitsinclude analog memory devices, and digital electrically programmablememory cells (including cells that may be set into one of more than twopossible states, reflecting a multiple-bit data value). From aconstruction standpoint, such variations include other arrangements ofthe floating-gate device, including polysilicon-to-polysiliconfloating-gate capacitors, polysilicon-to-active capacitors, and thelike, and including floating-gate devices that are programmable by othermechanisms besides Fowler-Nordheim tunneling. Examples of suchalternative structures are described in U.S. Patent ApplicationPublication No. US 2013/0221418 and U.S. Pat. No. 8,779,550, bothcommonly assigned herewith, and in Ahuja et al., “A Very High Precision500-nA CMOS Floating-Gate Analog Voltage Reference”, J. Solid-StateCirc., Vol. 40, No. 12 (IEEE, December 2005), pp. 2364-72, all suchreferences incorporated herein by reference. It is contemplated thatthose skilled in the art having reference to this specification will bereadily able to realize these, and other, variations as appropriate forparticular circuit applications, without undue experimentation.

Also as shown in FIGS. 2a, 2c, and 2d and as described above, conductorelement 21 is formed as a metal pad at the surface of integrated circuit20 to be in electrical contact with polysilicon element 36 (i.e.,floating-gate electrode 22). In this example, first interlevel insulator40 a overlies polysilicon element 36, with conductive plug 41 a disposedin a via in that insulator 40 a and in contact with polysilicon element36 as shown. Conductive pad 42, formed of a metal (i.e., in the firstmetal level) or another conductor material, is disposed at the surfaceof first interlevel insulator 40 a and in contact with plug 41 a.Similarly, second interlevel insulator layer 40 b overlies firstinterlevel insulator 40 a, with conductive plug 41 b formed in a viathrough layer 40 b to contact conductive pad 42. Conductor element 21 inthis example is formed at the surface of second interlevel insulator 40b, at a location overlying and in contact with conductive plug 41 b. Assuch, conductor element 21 is in electrical contact with polysiliconelement 36, i.e. floating-gate electrode 22, by the series connectionsof plugs 41 a, 41 b and pad 42. While plugs 41 a, 41 b and pad 42 areall illustrated as directly overlying one another in the example of FIG.2d , it is of course contemplated that plugs 41 a, 41 b may be laterallyseparated from one another (i.e., contacting conductive pad 42 atdifferent locations along its length) if desired. In addition, while theexample of FIG. 2d shows two metal levels (pad 42 and element 21) beingused for this structure, it is of course contemplated that additionalmetal levels may be included in the various layers between conductiveelement 21 and polysilicon element 36, as known in the art. In anyevent, according to these embodiments, conductive element 21 is exposedat the surface of the integrated circuit, either above the top insulatorlayer (layer 40 b in the example of FIG. 2d ), or alternatively exposedthrough an opening in a protective overcoat or other overlying insulatorlayer.

One or more reference conductive elements 25 are also provided at thesurface of the integrated circuit. These reference conductive elements25 are contemplated to be one or more metal features formed in the sameconductive layer as conductive element 21, and near to but spaced apartfrom conductive element 21. In the example shown in FIG. 2a , referenceconductive elements 25 are in the form of metal pads that aresignificantly larger in area than conductive element 21; in thisarrangement, it is contemplated that these multiple metal pads willtypically surround conductive element 21. According to anotherembodiment as shown in FIG. 2c , a single reference conductive element25′ is in the form of a metal ring that encircles conductive element 21.In any case, reference conductive elements 25, 25′ (collectivelyreferred to as reference conductive elements 25) are each separated fromconductive element 21 by some distance SEP, and as such are not indirect electrical contact with conductive element 21 (nor in contactwith any other node of the analog floating-gate circuit, for thatmatter). It is contemplated that one or more reference conductiveelements 25 that surround conductive element 21 on all sides willprovide repeatable and consistent results. Each reference conductiveelement 25 is in contact with a conductor or a semiconductor region thatwill, in operation, be at a reference voltage such as ground. As in thecase of conductive element 21, each reference conductive element 25 willbe exposed at the surface of the integrated circuit, either above thetop insulator layer (layer 40 b in the example of FIG. 2d ), oralternatively exposed through an opening in a protective overcoat orother overlying insulator layer. If the latter, it is believed to bepreferable that the entire region of the surface of the integratedcircuit containing conductive element 21 and nearby reference conductiveelements 25 be exposed within a single opening in the protectiveovercoat.

According to these embodiments, as will be described in further detailbelow, the operation of atmometer sensor 5 is based on the electricaleffects of the evaporation of moisture dispensed at the surface ofintegrated circuit 20 by moisture dispenser 30, in response to a signalfrom control circuitry 35. This moisture at the surface of integratedcircuit 20 both in liquid form (e.g., moisture droplet M in FIG. 2d ) asdispensed, and also in the form of the moist air produced by theevaporation of that moisture, provides a conduction path betweenconductor element 21 and reference conductor elements 25. Charge fromfloating-gate electrode 22 will conduct via this conduction path to thereference voltage at reference conductive elements 25 until the moisturehas evaporated to the extent that the conduction path disappears. Thedischarge of floating-gate electrode 22 caused by this conduction willbe reflected in changes in the drain current ID conducted by transistor24 under bias, from which the evaporation rate can be inferred. Morespecifically, because charge will no longer be conducted fromfloating-gate electrode 22 once moisture has evaporated from thesurface, the time at which the drain current ID stabilizes following thedispensing of the moisture will indicate the evaporation rate at thelocation of atmometer sensor 5, under the current environmentalconditions.

Referring back to FIG. 1b , in order for atmometer sensor 5 to providerepeatable measurements of evaporation rate, moisture dispenser 30should be capable of repeatably dispensing moisture droplets, repeatablein the sense that the droplet does not vary appreciably in size orcenter location from measurement to measurement. One type of apparatussuitable for use as moisture dispenser 30 in these embodiments is anink-jet printer head type of dispenser, as this type of dispenser iscapable of dispensing liquid droplets of consistent size to preciselocations; it is also contemplated that other conventional types ofdispensing apparatuses, such as controllable atomizers and the like, mayalternatively be used. Good repeatability also requires that thedispensed moisture be of consistent conductivity, and preferablyrelatively low conductivity to facilitate accuracy in the evaporationrate measurement. It is contemplated that distilled water will besuitable for use as the medium dispensed by moisture dispenser 30 inthese embodiments; additional precision can be obtained by measuring theconductivity of the distilled water or other medium prior to loadingmoisture dispenser 30.

As mentioned above, optional heater 31 may be implemented in atmometersensor 5 to dry the surface of integrated circuit 20 betweenmeasurements. It is contemplated that various types of elements may beused as heater 31, including one or more polysilicon or diffusedresistors in integrated circuit 20 itself, or a heating element externalto integrated circuit 20 within atmometer sensor 5 and under the controlof control circuitry 35. If implemented, however, heater 31 should becontrolled by control circuitry 35 to not alter the environmentalconditions at the surface of integrated circuit 20 at the time thatmeasurements are made; as such, it is contemplated that heater 31 willbe only temporarily activated between measurements, perhaps with timeallotted for atmometer sensor 5 to return to an equilibrium conditionrelative to the surrounding environment.

FIG. 3a illustrates an equivalent circuit for the effects of moistureand evaporation at the surface of analog floating-gate integratedcircuit 20 of FIGS. 1b and 2a through 2d . As shown in FIG. 3a , thisequivalent circuit includes storage capacitor 26 receiving gate voltageVG, and transistor 24 having its drain biased at voltage VD and itssource at ground. The conduction path between conductive element 21 andreference conductive elements 25 provided by the dispensed moisture andthe surrounding moist air from evaporation may be modeled in thisequivalent circuit by the series connection of switch 50 andtime-varying variable resistor 52, where the resistance of variableresistor 52 is increased by control function 54 over time. FIG. 3bqualitatively illustrates the operation of this equivalent circuit inthe measurement of evaporation rate. The initial condition of thecircuit of FIG. 3a , prior to time t₀ in FIG. 3b , has drain voltage VDand gate voltage VG both above the threshold voltage of transistor 24,while a reference voltage, for example ground, biases each of referenceconductor elements 25. A high fraction (typically on the order of 90%)of the gate voltage VG will capacitively couple to floating-gateelectrode 22, turning on transistor 24 and resulting in drain currentID₀ being conducted prior to time t₀. At this point in the operation,because the surface of integrated circuit 20 is dry, no conduction pathis present between conductor element 21 and reference conductor elements25. As such, floating-gate electrode 22 does not discharge to ground,and drain current ID remains constant at the level ID₀.

At time t₀, moisture is dispensed at the surface of integrated circuit20 by moisture dispenser 30, in the form of one or more moisturedroplets M. This moisture presents a conduction path between conductorelement 21 and one or more of reference conductor elements 25. Referringto the equivalent circuit of FIG. 3a , switch 50 closes at time t₀, suchthat floating-gate electrode 22 is coupled to ground via resistor 52.Charge present at floating-gate electrode 22 will then conduct fromconductive element 21, through the conduction path of moisture droplet Mand the surrounding moist air due to evaporation of droplet M, to theground potential at reference conductor elements 25. As charge isremoved from floating-gate electrode 22, the gate potential oftransistor 24 drops, causing drain current ID to drop following time t₀,as evident in FIG. 3 b.

Beginning at time t₀, the dispensed moisture will evaporate at a ratedepending on the current environment in the vicinity of the surface ofintegrated circuit 20. As this moisture evaporates, the conduction pathbetween conductor element 21 and reference conductor elements 25 willbecome more resistive, which is modeled in the equivalent circuit ofFIG. 3a by control function 54 causing the resistance of variableresistor 52 to increase following time t₀. Upon sufficient evaporation,the resistance of variable resistor 52 will effectively become infiniteand no conduction path due to moisture will remain between conductorelement 21 and reference conductor elements 25, at which time the lossof charge from floating-gate electrode 22 ceases. Switch 50 can beconsidered to be open at this point in time, shown in FIG. 3b as timet_(evap). Once floating-gate electrode 22 is no longer being discharged,drain current ID stabilizes at its then-current value, and will remainconstant as long as transistor 24 is under the same bias conditions.

According to these embodiments, the evaporation rate can be calculatedfrom the time elapsed between the dispensing of moisture, at time t₀,and the stabilizing of drain current ID to a steady-state condition, attime t_(evap). The steady-state level of drain current ID that isreached at time t_(evap) is not particularly relevant to thedetermination of evaporation rate, but is more related to the amount ofmoisture (size of droplet M) dispensed at the surface of integratedcircuit 20 (i.e., more water will reduce the initial resistance ofvariable resistor 52, and will also extend the time required to reachtime t_(evap)). Calibration of the behavior of atmometer sensor 5 to anindependent measurement of evaporation rate, for example by aconventional atmometer (which itself is calibrated), will enabledetermination of an evaporation rate, assuming the repeatable dispensingof moisture from measurement to measurement.

Referring to the equivalent circuit of FIG. 3a , the conduction ofcharge from floating-gate electrode 22 will follow an RC time constant,in which the initial value of the resistive component will depend on thesize of moisture droplet M relative to the separation SEP betweenconductive element 21 and reference conductive element(s) 25. For theexample of a floating-gate capacitance of on the order of 100 pF, aconduction path having a resistance of on the order of 10 GΩ wouldprovide a time constant of about 1 second. Because the resistance of dryair (i.e., before moisture is dispensed) is much greater than 10 GΩ, noappreciable conduction of charge from floating-gate electrode 22 willoccur, as evident in FIG. 3b prior to time t₀. On the other hand, theresistivity of liquid water with impurities is in the MS range, whichwould rapidly decay the charge from floating-gate electrode 22, wellwithin one second in this example. It is contemplated that precisemeasurements of evaporation rate will be more easily attained if thetime constant of the charge decay is on the order of seconds. Thisslower decay can be attained by a sufficiently large distance SEPbetween conductive element 21 and reference conductor element(s) 25 thata single moisture droplet M (e.g., of a radius on the order of 100μ)covering conductive element 21 will not extend to reference conductiveelement 25. This results in at least part of the conduction path beingconstituted by the moist air from evaporation of the droplet, which willbe more resistive than the liquid droplet itself. Accordingly, it iscontemplated that many implementations will select a distance SEPbetween reference conductive elements 25 and conductive element 21 thatis sufficiently large so that the dispensed moisture droplet M willcontact only conductive element 21 and not contact reference conductiveelements 25. Alternatively, if a more rapid decay time is desired, ashorter distance SEP may be implemented so that the dispensed droplet Mcontacts both conductive element 21 and reference conductive elements25.

Referring now to FIG. 4, the overall operation of atmometer system 5 ofFIG. 1b in calculating an instance of an evaporation rate will now bedescribed. As discussed above in the context of FIGS. 1a and 1b , it iscontemplated that the control of the operations of atmometer system 5and the execution of various calculations based on the results of thatoperation may be performed on a scheduled or automated basis, forexample in a networked system of sensors and controllers deployed in thearea or environment to be monitored. As such, it is contemplated thatmany implementations may configure control circuitry 35 within atmometersensor 5 itself, or MCU 2 in node N for the particular sensor 5, or somecombination of the two, to have the necessary and appropriatecomputational capacity to carry out these operations and calculations,and that such implementation will be readily apparent to those skilledin the art having reference to this specification. In any case, whilethe following description will be referring to the various components ofatmometer system 5 and node N, those skilled in the art will recognizethat such description is presented by way of example only, and thatthese embodiments may be realized over a wide array of architectures andsystems, without undue experimentation.

Prior to beginning a measurement of evaporation rate, optional heatingprocess 60 may be performed if desired. In process 60, heater 31 (FIG.1b ) is actuated by control circuitry 35 to heat the surface ofintegrated circuit 20 so as to evaporate any residual moisture at thatsurface from a previous measurement, and to evaporate any condensationon that surface that may have formed. As described above, heatingprocess 60 should not affect the environmental conditions at the surfaceof integrated circuit 20 during the evaporation rate measurement.

In process 62, floating-gate electrode 22 is neutralized by tunnelcapacitors 28 p, 28 n so as to have no residual charge. Process 62 maybe performed by applying a pulse of either or both an appropriatenegative voltage (e.g., on the order of −10 volts) to terminal TN and anappropriate positive voltage (e.g., on the order of +10 volts) atterminal TP, both relative to a ground reference voltage applied to allother nodes (gate G, source S, drain D). This operation is intended toremove any residual positive charge and residual electrons fromfloating-gate electrode 22. Once neutralized in process 62,floating-gate electrode 22 is at a known neutral state and may be thencharged by the application of a bias voltage at gate G.

In process 64 in this embodiment, control circuitry 35 applies biasvoltages to gate G, to drain D of transistor 24, and to referenceconductor elements 25. For the example of an n-channel transistor 24, apositive polarity drain voltage VD above the threshold voltage oftransistor 24 will be applied to drain D, and a positive polarity gatevoltage VG that capacitively couples to charge floating-gate electrode22 above the threshold voltage of transistor 24 will be applied to gateG. A reference voltage such as ground, for the case of a positivepolarity gate voltage applied to floating-gate electrode 22 and thusconductor element 21, is also applied to reference conductor elements 25in process 64. As a result of process 64, transistor 24 will conduct aconstant drain current ID₀, with the level of that current depending onthe particular bias conditions and transistor parameters.

In process 66, control circuitry 35 issues a control signal to moisturedispenser 30 to cause it to dispense moisture at the surface ofintegrated circuit 20, specifically to dispense moisture contactingconductor element 21. As discussed above, it is contemplated thatdispensation process 66 may deposit either a single droplet or multipledroplets over the surface of integrated circuit 20. In any case, asmentioned above, it is beneficial for the size of moisture droplets Mdispensed in process 66 to be controllable so as to be consistent overtime, allowing calibration of atmometer sensor 5 to repeatably providean evaporation rate measurement.

In process 68, control circuitry 35 monitors the drain current ID asconducted by transistor 24 to determine the time elapsed after thedispensing of moisture at the surface in process 66 until drain currentID reaches a stable level. As discussed above, this time elapsed toreach steady-state equilibrium indicates the rate at which moistureevaporates at the surface of integrated circuit 20 under currentenvironmental conditions. This elapsed time is then applied tocomputational circuitry, such as within control circuitry 35 ofatmometer sensor 5 or by ALU 10 in MCU 2 of node N, to calculate anevaporation rate in process 70. As discussed above, it is contemplatedthat this calculation will apply previously determined calibration datathat correlates elapsed time values as monitored in process 68 to anevaporation rate, given such parameters of the moisture dispensed inprocess 66 as droplet size, droplet quantity, and conductivity of thedispensed water.

Upon completion of calculation process 70, the evaporation rate detectedby atmometer sensor 5 may then be communicated to the appropriatedestination host computer, or local or remote data base, for example bywireless or other communications carried out by transceiver 4 of node N,in process 75. These results may be communicated in process 75immediately on a real time basis, or alternatively may be stored locallyat atmometer sensor 5 or at node N for later communication or retrieval.Further in the alternative, as shown in FIG. 4, the elapsed time resultsof process 68 may be communicated in process 75, in which case theevaporation rate calculation of process 70 will be performed by a remotecomputer.

According to another embodiment in which atmometer sensor 5 is operatingas a relative humidity sensor, the calculated evaporation rate is usedto calculate a relative humidity of the environment at integratedcircuit 20. As described in the above-incorporated Gu et al. article andits supporting information, the evaporation of a droplet of water can beconsidered as the diffusion of water from the surface of that droplet(i.e., where saturated) into the surrounding atmosphere in which thewater vapor concentration is smaller. This gives rise to a diffusionequation:

$\frac{\partial c}{\partial t} = {D\; \Delta \; c}$

where D is the diffusivity of water vapor in the air and c is the watervapor concentration. As known in the art and as described in the Gu etal. article, the diffusivity D is dependent on the ambient temperature,and may be expressed for typical conditions as

D(T)=0.171T+0.28

This diffusion equation allows for the derivation of a characteristictime τ_(f) for the “disappearance” of a water droplet, due toevaporation, as dependent on the radius of the water droplet, thediffusivity D(T) as a function of temperature, the relative humidity H(as a fraction), and the saturated water vapor concentration c_(V) atthe surface of the water droplet:

$\tau_{f}\text{∼}\frac{\rho \; R^{2}}{{D(T)}( {1 - H} )c_{V}}$

where ρ is the density of water. Because the characteristic time τ_(f)amounts to an alternative expression for the rate at which the waterdroplet evaporates, relative humidity H can be calculated from theevaporation rate calculated in process 70 based on the measurementsobtained by atmometer sensor 5.

According to this alternative embodiment, therefore, process 72 isperformed to measure or obtain measurement data of the ambienttemperature in the vicinity of atmometer sensor 5. As mentioned above,an additional sensor 5 may be provided within node N to locally obtainthis temperature measurement, or alternatively the temperaturemeasurement may be communicated to node N or to such other computationalcircuitry in the network performing these calculations, in process 72.Upon obtaining the evaporation rate calculated in process 70 and thetemperature measurement in process 72, this computational circuitry suchas ALU 10 in node N or elsewhere in the network then calculates therelative humidity in process 74, for example by using the equationsdiscussed above. The results of relative humidity calculation process 74are then forwarded on to the appropriate destination as described above,in process 75.

As will be apparent from this description to those skilled in the art,these embodiments can provide important advantages in the measurementand evaluation of evaporation rate, more particularly in enabling therapid and frequent measuring of evaporation rate in a repeatable andreliable manner. The ability to implement the sensing mechanism in anintegrated circuit realization within an atmometer system iscontemplated to facilitate the efficient and inexpensive deployment of alarge number of these sensors in an environment, such as an agriculturalfield, and in a machine-to-machine (M2M) networked system so that themeasurements can be collected in an automated manner with the resultscommunicated and collected over a wide-area network. In addition, othercalculations based on the measured evaporation rate, such as relativehumidity, can be readily carried out, also in an automated and networkedmanner if desired.

While one or more embodiments have been described in this specification,it is of course contemplated that modifications of, and alternatives to,these embodiments, such modifications and alternatives capable ofobtaining one or more the advantages and benefits of this invention,will be apparent to those of ordinary skill in the art having referenceto this specification and its drawings. It is contemplated that suchmodifications and alternatives are within the scope of this invention assubsequently claimed herein.

What is claimed is:
 1. A method of measuring evaporation rate,comprising: applying a drain-to-source voltage to a floating-gatetransistor in an integrated circuit; capacitively coupling a voltage toa floating-gate electrode in the floating-gate transistor, to establisha gate-to-source voltage at that transistor; then dispensing moisture ata surface of the integrated circuit at which an electrode in electricalcontact with the floating-gate electrode and at least one referenceelectrode are exposed; then monitoring current conducted by thefloating-gate transistor to measure an elapsed time at which the currentstabilizes; and determining an evaporation rate responsive to themeasured elapsed time.
 2. The method of claim 1, further comprisingneutralizing the floating-gate electrode before capacitively couplingthe voltage.
 3. The method of claim 2, wherein neutralizing thefloating-gate electrode comprises biasing a first doped region servingas a first plate of a first tunnel capacitor, wherein a first portion ofthe floating-gate electrode serves as a second plate of the first tunnelcapacitor.
 4. The method of claim 3, wherein neutralizing thefloating-gate electrode further comprises biasing a second doped regionserving as a first plate of a second tunnel capacitor, wherein a secondportion of the floating-gate electrode serves as a second plate of thesecond tunnel capacitor; wherein the first and second doped regions areof opposite conductivity type from one another; and wherein the firstand second doped regions are biased with voltages of opposite polarityfrom one another as a result of the biasing.
 5. The method of claim 2,further comprising heating the surface of the integrated circuit beforeapplying the drain-to-source voltage and before capacitively couplingthe voltage to the floating-gate electrode.
 6. The method of claim 2,further comprising: measuring an ambient temperature in the vicinity ofthe integrated circuit; and determining a relative humidity valueresponsive to the evaporation rate and the measured ambient temperature.7. An integrated circuit comprising: source and drain doped regionsformed at a semiconductor surface of a body, and separated from oneanother by a channel region at the surface; a gate dielectric filmdisposed over the surface; a floating-gate electrode overlying the gateinsulator film, the floating-gate electrode having a first portionoverlying the channel region to serve as a gate electrode for atransistor comprised of the source and drain doped regions, and having asecond portion; a conductive plate disposed over the second portion ofthe floating-gate electrode and separated therefrom by a capacitordielectric film, to form a storage capacitor; a first conductor elementin electrical contact with the floating-gate electrode, and exposed tothe atmosphere at the surface of the integrated circuit; and one or morereference conductor elements exposed to the atmosphere at the surface ofthe integrated circuit near the first conductor element, and adapted tobe coupled to a reference voltage.
 8. The integrated circuit of claim 7,further comprising: a first doped region of a first conductivity typedisposed at the surface at a location underlying a third portion of thefloating-gate electrode, and separated therefrom by a tunnel dielectricfilm, to form a first tunnel capacitor.
 9. The integrated circuit ofclaim 8, further comprising: a second doped region of a secondconductivity type disposed at the surface at a location underlying afourth portion of the floating-gate electrode, and separated therefromby the tunnel dielectric film, to form a second tunnel capacitor.
 10. Anatmometer system, comprising: an integrated circuit, comprising: afloating-gate electrode; a metal-oxide-semiconductor (MOS) transistorhaving source and drain regions disposed in a semiconductor surface ofthe integrated circuit and separated from one another by a channelregion, wherein a first portion of the floating-gate electrode isdisposed over the channel region to serve as a gate electrode of the MOStransistor; a storage capacitor having a first plate comprised of asecond portion of the floating-gate electrode, and a second plateseparated from the first plate by a dielectric film; a first conductorelement in electrical contact with the floating-gate electrode, andexposed at a surface of the integrated circuit; and one or morereference conductor elements disposed at the surface of the integratedcircuit near the first conductor element, and coupled to a referencevoltage node; and control circuitry, coupled to the drain region of theMOS transistor and to the second plate of the storage capacitor, thecontrol circuitry being configured to bias the gate of the MOStransistor, output a moisture dispensing control signal, and measurecurrent conducted by the MOS transistor.
 11. The atmometer system ofclaim 10, wherein the integrated circuit further comprises a firsttunnel capacitor having a first plate comprised of a third portion ofthe floating-gate electrode, and a second plate separated from the firstplate by a dielectric film and coupled to the control circuitry.
 12. Theatmometer system of claim 11, wherein the integrated circuit furthercomprises: a second tunnel capacitor, having a first plate comprised ofa fourth portion of the floating-gate electrode, and a second plateseparated from the first plate by a dielectric film and coupled to thecontrol circuitry; wherein the second plate of the first tunnelcapacitor comprises a doped semiconductor region of a first conductivitytype; and wherein the second plate of the second tunnel capacitorcomprises a doped semiconductor region of a second conductivity type.13. The atmometer system of claim 12, wherein the control circuitrycomprises: processor circuitry configured to execute a sequence ofoperations comprising: biasing the second plates of each of the firstand second tunnel capacitors to neutralize a potential of the electrode;then biasing the transistor by applying a voltage to the second plate ofthe storage capacitor and a drain-to-source voltage across thetransistor; then controlling the moisture dispenser to dispense moistureat the surface of the integrated circuit; and measuring currentconducted by the transistor over an interval of time following thecontrolling operation to detect an elapsed time at which the currentstabilizes.
 14. The atmometer system of claim 13, further comprising acommunications transceiver coupled to the control circuitry andconfigured to communicate signals responsive to the elapsed time. 15.The atmometer system of claim 14, wherein the sequence of operationsfurther comprises: determining an evaporation rate responsive to themeasured current; receiving a temperature measurement; and determining arelative humidity responsive to the evaporation rate and the temperaturemeasurement.
 16. The atmometer system of claim 10, further comprising aheater disposed near or in the integrated circuit and coupled to thecontrol circuitry.
 17. The atmometer system of claim 16, wherein theheater comprises a polysilicon resistor.